Noise reduction in digitizer system

ABSTRACT

A method for noise reduction in a digitizer, the digitizer comprising a plurality of detecting elements for detecting an electromagnetic signal at one of a number of predetermined frequencies: the method comprising: sampling at least two of said detecting elements substantially simultaneously to obtain outputs therefrom, and reducing the output on one of said two elements in accordance with the output on the other of said elements.

RELATIONSHIP TO EXISTING APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication No. 60/547,772, filed Feb. 27, 2004, the contents of whichare hereby incorporated by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to noise reduction in a system comprisinga digitizer and, more particularly, but not exclusively to noisereduction in a system comprising a digitizer associated with a displayscreen.

U.S. Pat. No. 6,690,156 “Physical Object Location Apparatus and Methodand a Platform using the same” assigned to N-trig Ltd, and U.S. patentapplication Ser. No. 10/649,708 “Transparent Digitizer” also assigned toN-trig Ltd, describe a positioning device capable of detecting multiplephysical objects, preferably styluses, located on a flat screen display.One of the preferred embodiments in both patents describes a systembuilt of transparent foils containing a matrix of vertical andhorizontal conductors. In one embodiment the stylus includes a passiveresonance circuit, which is triggered by an excitation coil thatsurrounds the foils. The stylus is excited at a predetermined range offrequencies depending on the capacitance and inductance of the resonantcircuit. Other embodiments may include a different kind of EM stylus.The exact position of the stylus is determined by processing the signalsthat are sensed by the matrix of horizontal and vertical conductors.

Existing digitizer systems use several noise removal methods to improvethe detection precision. For example the received signal is processedthrough a band pass filter leaving a window of frequencies including thestylus frequency. The filtered signal may then be passed through aFourier transform selecting the single frequency of the stylus.

Elements that induce an equal amount of noise on each conductive lineregardless of the line location may then be eliminated through the useof differential amplifiers. For example, objects that are far enoughfrom the sensor will have the same effect on all the sensor lines.

There are other examples of noise reduction methods that do noteliminate noise at the stylus frequency.

Using the various prior art systems, much of the noise is removed, butone element of noise necessarily remains because it cannot be identifiedand filtered out and that is noise that is at the same frequency as thestylus.

Preferred Application

The preferred application to which the embodiments to be describedhereinbelow are applicable is a transparent digitizer for a mobilecomputing device that uses a flat panel display (FPD) screen. Thedigitizer detects the position of one stylus at a very high resolutionand update rate. The stylus is used for pointing, painting, writing(hand write recognition) and any other activity that is typical for astylus. The digitizer supports full mouse emulation. As long as thestylus hovers above the FPD, a mouse cursor follows the stylus position.Touching the screen stands for left click and a special switch locatedon the stylus emulates right click operation.

The application may utilize a passive EM stylus. External excitationcoils that surround the sensor are utilized to energize the stylus.However, other versions may include an active stylus, battery operatedor wire connected, which does not require external excitation circuitry.

In one application the electromagnetic object responding to theexcitation is a stylus. However, other embodiments may include otherphysical objects comprising a resonant circuit or active oscillators,such as gaming pieces. Applications describing gaming tokens comprisingresonant circuits are described in U.S. Pat. No. 6,690,156 (“physicalobject location apparatus and method and a platform using the same”).

In the preferred application, the digitizer can detect simultaneous andseparate inputs from an electromagnetic stylus and a user finger. Hence,it is capable of functioning as a touch detector as well as detecting astylus. However, other embodiments may include a digitizer capable ofdetecting only an electromagnetic stylus.

In a preferred application, the stylus supports full mouse emulation.However, in different applications the stylus could support additionalfunctionality such as an Eraser, change of color, etc. In otherembodiments the stylus could be pressure sensitive and changes itsfrequency or changes other signal characteristics in response to userpressure.

In a preferred application, the mobile device is an independent computersystem having its own CPU. In different embodiments the mobile devicemight only be a part of system such as a wireless mobile screen for aPersonal Computer.

In a preferred application, the digitizer is integrated into the hostdevice on top of the FPD screen. In additional application thetransparent digitizer can be provided as an accessory that could beplaced on top of a screen. Such a configuration can be very useful forlaptop computers, which are already in the market in very large numbers.Such systems can turn a laptop into a powerful device that supports handwriting, painting or any other operation enabled by the transparentdigitizer.

In a preferred application, the digitizer supports one stylus. However,in different applications more than one stylus may operatesimultaneously on the same screen. Such a configuration is very usefulfor entertainment application where multiple users can paint or write tothe same paper-like screen.

In one application, the digitizer is implemented on a set of transparentfoils. Alternatively such a digitizer may be implemented using either atransparent or a non-transparent sensor. One example is a Write Paddevice, which is a thin digitizer that is placed below normal paper. Inthis example, the stylus combines real ink with electro magneticfunctionality. The user writes on the normal paper and the input issimultaneously transferred to a host computer to store or analyze thedata.

An additional example of a non-transparent sensor is an electronicentertainment board. The digitizer, in this example, is mounted belowthe graphic image of the board, and detects the position and identity ofgaming figures that are placed on top the board. The graphic image inthis case is static, but it could be manually replaced from time to time(such as when switching to a different game).

In some applications a non-transparent sensor could be integrated in theback of a FPD. One example for such an embodiment is an electronicentertainment device with a FPD display. The device could be used forgaming, in which the digitizer detects the position and identity ofgaming figures. It could also be used for painting and/or writing inwhich the digitizer detects one ore more styluses. In most cases, aconfiguration of non-transparent sensor with a FPD will be used whenhigh performance is not critical for the application.

Technical Description

Transparent Digitizer

A preferred digitizer allows for the location and identification ofphysical objects, such as styluses and users fingers. Identifying thelocation of the physical objects is sensed by an electro magnetictransparent digitizer that is mounted on top of a display. Thetransparent digitizer is described in U.S. Pat. No. 6,690,156 anddetailed in U.S. patent application Ser. No. 10/649,708.

The various components and functionality manner of the transparentdigitizer are as follows.

-   -   Sensor

In the preferred digitizer, the sensor is a grid of conductive linesmade of conductive materials, such as ITO or conductive polymers,patterned on a transparent foil or substrate. For further informationplease refer to U.S. patent application Ser. No. 10/649,708,sub-heading: “Sensor”, the contents of which are hereby incorporatedherein by reference.

-   -   Frontend

In the preferred digitizer the Front end is the first stage where sensorsignals are processed. Differential amplifiers amplify the signals andforward them to a switch, which selects the inputs to be furtherprocessed. The selected signal is amplified and filtered by a filter &amplifier prior to sampling. The signal is then sampled by an A2D andsent to a digital unit via a serial buffer. For further informationplease refer to U.S. patent application Ser. No. 10/649,708, under theheading “Front end”, the contents of which are hereby incorporated byreference herein.

-   -   Digital unit

In the preferred digitizer the digital unit functions as follows: Thefront-end interface receives serial inputs of sampled signals from thevarious front-ends and packs them into parallel representation. Aprocessing unit, such as a DSP core or a processor, which performs thedigital unit processing, reads the sampled data, processes it anddetermines the position of the physical objects, such as stylus orfinger. The Digital unit can be embedded in an ASIC component. Thecalculated position coordinates are sent to the host computer via link.For further information please refer to subheading: “Digital unit” inU.S. patent application Ser. No. 10/649,708, the contents of which arehereby incorporated by reference.

-   -   Detector

The detector consists of the digital unit and the Front end.

Detection of Stylus

The preferred digitizer utilizes a passive electromagnetic (EM) stylus.The stylus comprises two main sections; the first section is an energypick-up circuit and the second section is an active oscillator which iscoupled to the stylus tip. An external excitation coil that surroundsthe sensor supply energy to the energy pick up circuit. The receivedenergy is transferred to the active oscillator through a rectifyingcomponent such as a diode bridge. The exact position of the stylus isdetermined by the detector, which processes the signals sensed by thesensor. In the preferred embodiment only the electric wave of theelectromagnetic signal generated by the stylus, is utilized; However,other embodiments may utilize the magnetic portion in addition orinstead of the electric wave. For further information please refer toU.S. patent application Ser. No. 10/649,708 assigned to N-trig, and U.S.provisional patent application “Electromagnetic Stylus for a DigitizerSystem” filed December 2004, also assigned to N-trig, the contents ofboth applications are hereby incorporated by reference.

In the preferred digitizer, the basic operation cycle consists ofwindowing, FFT/DFT, peak detection, interpolation, filtering andsmoothing. For further information please refer to U.S. patentapplication Ser. No. 10/649,708, sub-title:“Algorithms”.

Noise Sources

There may be a variety of noise sources in the stylus frequency range.The most common signals interfering with the stylus signals are signalsthat originate from conductive objects, such as a user finger, touchingthe screen. FIG. 1 is an electrical equivalent of a user finger touchingone of the digitizer's antennas. When the user touches an antenna 11 acapacitance 12 is formed between the finger and the sensor conductors.

The noise situation is best explained with respect to finger inducednoise signals.

There are two main scenarios that cause finger induced signals

1. When the system is not connected to the common ground, electricalnetwork vibrations lead to system oscillations 10 in reference to theground. Since the user's body is not oscillating, the capacitance 12between the user's finger and the system induces leakage current 13through the user's finger to the ground.

2. When the users body is subjected to electromagnetic interferencesfrom the environment, it, and any associated finger, oscillates inreference to the system; as a result a leakage current is induced fromthe user's finger to the conductive antennas.

In both cases, the digitizer senses a leakage current originating fromthe user touching the sensor. When the leakage current induces a signalthat is at the same frequency of the stylus, the leakage current can bemistaken for a stylus signal.

A second possible source of noise is the electronic components withinthe system, which radiate at many frequencies. These components mayinduce noise signals at the stylus frequency; thus interfering withstylus detection. Electronic devices placed in proximity to the system,such as cellular phones, may also radiate in frequencies that interferewith the stylus detection.

FIG. 2 is an example of a noise and stylus affecting the sensor at thesame time. In this case the noise source is the users finger touchingthe screen. The sensor 20 comprises a matrix of conductive lines. Whenstylus tip 21 is present at the surface of the sensor it affects theantennas in its proximity. One or more antennas in proximity to thestylus may suffer noise signals induced by a finger 22 touching thesensor. For example, antenna 23 exhibits signals induced by both stylus21 and finger 22.

Erroneous Stylus Detection

In a digitizer of this kind, the stylus detection comprises twodetection steps. The first step is to find the antenna exhibiting themaximum stylus signal. The second step is calculating the stylusposition by interpolating the signals on the maximum signal antenna andits surrounding antennas.

A digitizer system designed to detect an electromagnetic stylus maysuffer from two kinds of problems. The first kind is when the unwantedsignals are stronger then the stylus signals, thus interfering with thefirst detection step. In this case the digitizer system should sampleand employ the noise removal algorithm on all the antennas in order toreveal the antenna exhibiting the maximum stylus signal.

Reference is now made to FIG. 3, which describes a case when a user'sfinger 22 touching the screen induces a stronger signal 33 than thestylus 21, causing the digitizer to mistake the finger for the stylus.As a result the digitizer chooses the wrong antennas for interpolation.

The second kind of problem is when the stylus signal is stronger thanthe finger-induced signal. However, an error in the stylus detection maystill occur during the interpolation step of the detection. FIG. 4, towhich reference is now made, describes a case when a users finger 22touching the screen induces a signal that causes the digitizer tomiscalculate the stylus position 34. The user finger 22 induces a signalon one of the X axis antennas 31 while the stylus 21 is located closerto a different X axis antenna 32. The signal received on the stylusantenna 32 is weaker than the signal 33 received on the finger antenna31. Hence, the digitizer will miscalculate the stylus position 34.

The object of the present invention is to solve both cases and eliminatenoise above and below the level of the stylus signal.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, a noise reduction system devoid of the abovelimitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod for noise reduction in a digitizer, the digitizer comprising aplurality of detecting elements for detecting an electromagnetic signalat one of a number of predetermined frequencies: the method comprising:

-   -   sampling at least two of the detecting elements substantially        simultaneously to obtain outputs therefrom, and    -   reducing the output on one of the two elements in accordance        with the output on the other of the elements.

Preferably, one of the at least two elements is selected as a candidatecarrier of a stylus signal and the other of the at least two elements isselected as a candidate carrier of mere noise.

The method preferably comprises detecting at each of the elements theone predetermined frequency and another arbitrary frequency.

The method preferably comprises utilizing the arbitrary frequency toestimate the amount of noise at the predetermined frequency.

The method preferably further comprises:

-   -   determining a ratio between signals of the one predetermined        frequency and the arbitrary frequency at the candidate carrier        of mere noise,    -   from the ratio and the arbitrary signal at the candidate carrier        of the stylus signal determining an amount of noise at the one        predetermined frequency, and    -   reducing a signal at the one predetermined frequency at the        candidate carrier of a stylus signal by the determined amount of        noise.

Preferably, the arbitrary frequency is selected as a frequency within apreset detection range having a relatively high noise.

Preferably, the digitizer is also for touch detection and the arbitraryfrequency is selected as a frequency already used for the touchdetection.

The method preferably further comprises deliberately generating noise atthe arbitrary frequency.

Preferably, the predetermined frequency is changed to a new frequencyduring use, the method comprising changing the arbitrary frequency froma frequency relatively close to the predetermined frequency to a secondfrequency relatively close to the new frequency.

Preferably, the candidate carrier of mere noise is selected from a groupof elements exhibiting more than a threshold amount of noise, as theelement in the group which is furthest away from a stylus previouslyknown location.

Alternatively, the candidate carrier of mere noise may be selected as anelement exhibiting a strongest noise signal as long as it is beyond adetermined distance from a stylus previously known location.

Alternatively, the candidate carrier of mere noise is arbitrarilyselected,

-   -   noise subtraction is carried out over a group of elements on the        basis of the selection being correct,    -   a resulting signal pattern over the group of elements is        analyzed, and    -   the arbitrary selection is allowed if the resulting signal        pattern is indicative of a correct selection, otherwise a new        arbitrary selection is made.

The method may comprise using as patterns indicative of a correctselection a first pattern indicative of a stylus at one of non-selectedelements, and a second pattern indicative of no stylus being present.

The method may comprise verifying the presence of a stylus beforecarrying out the reduction.

The method may comprise verifying the presence of a stylus,beforecarrying out the reduction, by comparing magnitudes at thearbitrary frequency and magnitudes at the predetermined frequency.

In one preferred embodiment:

-   -   the candidate carrier of mere noise is arbitrarily selected,    -   noise subtraction is carried out over a group of antennas on the        basis of the selection being correct,    -   a resulting signal pattern over the group of antennas is        analyzed, and    -   the arbitrary selection is rejected if the resulting signal        pattern is indicative of an incorrect selection, and a new        arbitrary selection is made.

The method may comprise using as a pattern indicative of an incorrectselection a pattern indicative of a stylus at or near the selectedcandidate.

The method may comprise using a complex proportion to compensate for atleast one of phase and magnitude differences between respective antennasduring the compensating.

The method may comprise sampling a group of antennas of an array,selecting at least one antenna which is least affected by the stylus andreducing respective outputs of at least some remaining antennas inaccordance with the output of the selected antenna.

The method may comprise using a plurality of arbitrarily selectedfrequencies to calculate the reduction.

The method may comprise using an average output of the plurality ofarbitrarily selected frequencies to calculate the reduction.

Preferably, the detecting elements are conductive detectors of a flatarray of the digitizer for digitizing signals of a movable object toindicate location of the object.

Preferably, the noise is at least in part a consequence of the presenceof a finger.

Preferably, the digitizer comprises a detection surface for carrying thedetecting elements, the detecting surface further being touch sensitive.

According to a second aspect of the present invention there is providedapparatus for noise reduction in a digitizer, the digitizer comprising aplurality of detecting elements for detecting an electromagnetic signalat one predetermined frequency of a plurality of predeterminedfrequencies: the apparatus comprising:

-   -   a sampler for sampling at least two of the detecting elements        substantially simultaneously to obtain outputs therefrom, and    -   a noise reduction unit for reducing the output on one of the two        detecting elements in accordance with the output on the other of        the detecting elements.

Preferably, one of the detecting elements is selected as a candidatecarrier of a stylus signal and the other of the detecting elements isselected as a candidate carrier of mere noise.

The apparatus may comprise a frequency detector for detecting outputs ateach of the detecting elements at the predetermined frequency andanother arbitrary frequency.

The apparatus may comprise

-   -   a ratio finder for determining a ratio between outputs at the        predetermined frequency and the arbitrary frequency at the        candidate carrier of mere noise,    -   and wherein the noise reduction unit is operable with the ratio        finder to:        -   determine from the ratio and the arbitrary signal at the            candidate carrier of the stylus signal determining an amount            of noise at the predetermined frequency, and        -   reduce the output at the predetermined frequency at the            candidate carrier of a stylus signal by the determined            amount of noise.

Preferably, the detecting elements are transparent conductors.

Preferably, the arbitrary frequency is selected as a frequency within apreset detection range having a relatively high noise.

Preferably, the arbitrary frequency is selected as a frequency alreadyused for finger detection.

The apparatus may comprise a noise generator for deliberately generatingnoise at the arbitrary frequency

Preferably, the predetermined frequencies are liable to change duringuse, the apparatus accordingly being configured to change the arbitraryfrequency from a frequency relatively close to a first predeterminedfrequency to a second frequency relatively close to a secondpredetermined frequency.

Preferably, the candidate carrier of mere noise is selected from a groupof detection elements exhibiting more than a threshold amount of noise,as the element in the group which is furthest away from a styluspreviously known location.

Preferably, the candidate carrier of mere noise is selected as anelement exhibiting a strongest noise signal as long as it is beyond adetermined distance from a stylus previously known location.

In an embodiment, the candidate carrier of mere noise is arbitrarilyselected or otherwise chosen, say using a selection algorithm,

-   -   noise subtraction is carried out over a group of elements on the        basis of the selection being correct,    -   a resulting signal pattern over the group of elements is        analyzed, and    -   the arbitrary selection is allowed if the resulting signal        pattern is indicative of a correct selection, otherwise a new        selection is made.

The apparatus may comprise using as patterns indicative of a correctselection a first pattern indicative of a stylus at one of non-selectedelements, and a second pattern indicative of no stylus being present.

The analysis may comprise determining a number of elements wherein anoutput exceeds a predetermined threshold.

In an alternative embodiment, the candidate carrier of mere noise isarbitrarily selected,

-   -   noise subtraction is carried out over a group of elements on the        basis of the selection being correct,    -   a resulting signal pattern over the group of elements is        analyzed, and    -   the arbitrary selection is rejected if the resulting signal        pattern is indicative of an incorrect selection, and a new        arbitrary selection is made.

The apparatus may comprise at least one of a phase compensator and amagnitude compensator using a complex proportion to compensate for atleast one of phase and magnitude differences between respective elementsduring the compensating.

Preferably, the elements comprise an array, the apparatus configured tosample at least some elements of the array, the noise reduction unitbeing configured to choose at least one element and reduce respectiveoutputs of at least some remaining elements in accordance with theoutput of another of the elements.

Preferably, the noise reduction unit is configured to use a plurality ofarbitrarily selected frequencies to calculate the reduction.

Preferably, the noise reduction unit is configured to use an averagenoise level of the plurality of arbitrarily selected frequencies tocalculate the reduction.

Preferably, the elements are conductive detectors of a flat array of thedigitizer for digitizing signals of a movable object to indicatelocation of the object.

Preferably, the movable object is a stylus.

Preferably, the stylus is an electromagnetic stylus.

Preferably, the stylus comprises an active oscillator and an energy pickup circuit.

Preferably, the stylus comprises a resonance circuit.

Preferably, the detecting elements are arranged in a grid array.

Preferably, the detecting elements are loop elements.

Preferably, the noise is at least in part a consequence of the presenceof a finger.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The materials, methods, andexamples provided herein are illustrative only and not intended to belimiting.

Implementation of the method and system of the present inventioninvolves performing or completing certain selected tasks or stepsmanually, automatically, or a combination thereof. Moreover, accordingto actual instrumentation and equipment of preferred embodiments of themethod and system of the present invention, several selected steps couldbe implemented by hardware or by software on any operating system of anyfirmware or a combination thereof. For example, as hardware, selectedsteps of the invention could be implemented as a chip or a circuit. Assoftware, selected steps of the invention could be implemented as aplurality of software instructions being executed by a computer usingany suitable operating system. In any case, selected steps of the methodand system of the invention could be described as being performed by adata processor, such as a computing platform for executing a pluralityof instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin order to provide what is believed to be the most useful and readilyunderstood description of the principles and conceptual aspects of theinvention. In this regard, no attempt is made to show structural detailsof the invention in more detail than is necessary for a fundamentalunderstanding of the invention, the description taken with the drawingsmaking apparent to those skilled in the art how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 is a simplified diagram showing a theoretical equivalent circuitof a finger on a digitizer surface;

FIG. 2 is a simplified diagram showing a sensor array of a digitizercircuit with a finger at one location and a stylus at another location;

FIG. 3 is a simplified diagram showing the sensor array of FIG. 2 inwhich an erroneous detection is made;

FIG.4 is a simplified diagram illustrating the sensor array of FIG. 2 inan alternative scenario in which an erroneous detection is made;

FIG. 5 is a simplified diagram illustrating a digitizer suitable for usewith a preferred embodiment of the present invention;

FIG. 6 is a simplified flow diagram illustrating a procedure inaccordance with a first preferred embodiment of the present invention;

FIG. 7 is a simplified flow diagram illustrating in greater detail theprocedure shown in FIG. 6, according to a further preferred embodimentof the present invention;

FIG. 8 is a simplified diagram illustrating a method of selecting anarbitrary frequency according to one preferred embodiment of the presentinvention;

FIG. 9 is a simplified diagram illustrating a method of selecting acandidate carrier of pure noise according to a preferred embodiment ofthe present invention;

FIG. 10 is a simplified flow diagram illustrating a second method ofselecting a candidate carrier of pure noise according to anotherpreferred embodiment of the present invention;

FIG. 11 is a simplified diagram illustrating operation of a digitizeraccording to an idealized embodiment of the present invention;

FIG. 12 is a simplified diagram illustrating operation of the digitizerof FIG. 11 in a less idealized case;

FIG. 13 is a simplified diagram illustrating operation of a digitizeraccording to a preferred embodiment of the present invention in whichthe method of FIG. 10 is used to identify a candidate carrier of purenoise;

FIG. 14 is a simplified diagram illustrating the operation of FIG. 10 inan alternative outcome;

FIG. 15 is a simplified diagram showing the equivalent components thatcontribute to noise detected by the digitizer;

FIG. 16 is a simplified diagram showing how the situation shown in FIG.15 varies for two noise sources; and

FIGS. 17 and 18 are two graphs illustrating signals sampled at antennasaccording to preferred embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present embodiments comprise a noise reduction system for stylusdetection in a digitizer. More particularly, the present embodimentscomprise a system for noise identification and reduction at the signalfrequency being used for detection of the stylus. It is noted that ifthe digitizer is combined with a touch detector, then the fingerinvolved in touch detection is likely to be a source of noisespecifically at the stylus detection frequency.

Such noise reduction specifically at the detection frequency may thusimprove the detection of an electromagnetic stylus in a digitizersystem. The digitizer is a computer associated input device capable oftracking user interactions via the stylus or other locatable objects. Ingeneral the digitizer is associated with a display screen, on which theresults of stylus detection may be displayed. The digitizer may furtherenable touch detection.

The present invention does not require that the digitizer be placeddirectly on the display screen. Rather, it is applicable both totransparent digitizers where the stylus is moved over the display screenand to other types of stylus, which are moved over tablets or paper orwhiteboards.

The present invention further applies to any kind of stylus or otherpointer device which has a detection frequency and not merely to apassive electromagnetic stylus. The noise reduction algorithm describedherein can be implemented on any digitizer system capable of trackingone or more electromagnetic styluses. The present invention isfurthermore applicable in systems designed to detect both stylus andtouch interactions, as will be explained hereinbelow.

The preferred embodiments are able to identify noise at the stylusfrequency, as will be explained below, and subtract the identified noisefrom the stylus signals, to leave the stylus signals as the only sourceof output at the stylus frequency.

It is noted that whilst the digitizer is designed to detect anelectromagnetic stylus, other conductive objects touching the screen mayinduce noise that can interfere with the stylus signal. Conventionalnoise removal methods, such as band pass filters and Fourier transformare often used to filter unwanted signals from the stylus signal.However, these methods can not remove unwanted signals at the samefrequency as the stylus, and, as mentioned above, a high percentage ofthe output at the desired frequency can be due to such induced noise.

The principles and operation of a noise reduction system according tothe present invention may be better understood with reference to thedrawings and accompanying description.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

Reference is now made to FIG. 5, which illustrates a simple digitizer.As mentioned, a digitizer is a device which detects the movement of anobject such as a stylus and converts it into a digital signal which canbe made available to software applications. A stylus or other pointingobject 110 is moved over a surface 112. Detectors 114 within the surface112 detect the current position of the pointer and send an output signalindicating that location to operating system 116 of a computer. Theoperating system ensures that the location information is made availableto currently active application 118, so that the application knows wherethe pointer is and can incorporate the position into its currentoperation.

Reference is now made to FIG. 6, which is a simplified diagramillustrating the basic principle behind noise reduction according to thepresent embodiments. The digitizer as shown in FIG. 5 has severaldetecting elements which between them detect signals from the stylus. Ina world without noise the detectors having the strongest signal would betaken as the location of the stylus. If two detectors give strongsignals then the location of the stylus may be taken as being somewherebetween them.

The real world has noise. Much of the noise is at frequencies other thanthe detection frequency being used by the stylus and can therefore beremoved simply by filtering or like techniques referred to in thebackground. However filtering cannot be used for the frequency beingused by the stylus. In the process shown in FIG. 6, two detectingelements are read at the same time at the frequency used by the stylus.Then the output obtained at one of the elements is subtracted from theoutput at the other element.

It will be appreciated that if one of the elements is an elementcurrently detecting a combination of stylus and noise output, and theother element is an element currently detecting only noise, then byobtaining the difference between the two signals one obtains a purestylus detection signal. The method therefore preferably includesinitial stage 120 of selecting an element as a candidate carrier of purenoise and a stage 122 of selecting another element as a candidatecarrier of the stylus signal. Several methods of selecting thesecandidates will be discussed hereinbelow. The outputs are measured atthe two elements in stage 124. As will be explained below, more than twocandidates may be used in practice. Finally in stage 126, the output atthe candidate carrier of the stylus signal is reduced in accordance withthe noise detection at the other element. It is pointed out that theabove applies to an “ideal” digitizer, where all the antennas andamplifiers are exactly the same. In such a case one can take the noiseoutput from one antenna and subtract it from a second antenna. Such asimplified solution can be implemented in cases such as that of FIG. 11which will be described below.

Reference is now made to FIG. 7, which is a simplified flow chartillustrating preferred outputs that can be used at the two carriers toobtain the necessary reduction. In stage 130, an arbitrary frequency isselected which is close to the frequency being used by the stylus sothat noise levels at the two frequencies may be expected to be related.In stage 132 the outputs of both elements are measured at the twofrequencies. In stage 134 a ratio is determined between the outputs atsaid two frequencies at the noise carrying element. In stage 136 thatratio is used to reduce the signal at the stylus frequency at the styluscarrying element.

In one preferred embodiment the arbitrary frequency is chosen using theprocedure shown in the flow chart of FIG. 8. A range of frequenciesaround the stylus frequency is monitored in stage 138. Then, in stage140, the arbitrary frequency is chosen as the frequency having thehighest output level.

In an alternative embodiment, the digitizer is additionally used fortouch detection and a given frequency is used for touch detection. Insuch an embodiment a frequency that can usefully be selected as thearbitrary frequency is the frequency already being used for touchdetection.

In one embodiment, it is possible to use the detection elements or otherparts within the digitizer systemto deliberately generate noise at thearbitrary frequency.

It is noted that in the case of a single stylus using a single frequencyat all times, the arbitrary frequency may be fixed. But in manyembodiments the stylus may change its frequency during use. For examplethe frequency may change in order to indicate particular states of thestylus. In such a case the arbitrary frequency may also be changed.

Reference is now made to FIG. 9, which is a simplified diagram showinghow a candidate carrier of mere noise may be selected. In FIG. 9 thestylus's most recent position is known, so it is assumed that thecurrent position is relatively close to that most recent position. Instage 142, measurements are taken to determine all of the detectingelements that are currently emitting an output at the arbitraryfrequency which exceeds a threshold signal level. There is thusestablished a group of elements emitting above the threshold. In stage144 the one element in the group that is furthest away from the stylus'smost recently known position is selected as the candidate carrier ofmere noise.

In a variation of stage 144, the candidate carrier of mere noise isselected as that element exhibiting a strongest noise signal, as long asit is beyond a determined distance from the previously known styluslocation.

Reference is now made to FIG. 10, which illustrates a more generalmethod of finding a candidate carrier of mere noise which does not relyon having a recent stylus location. The method of FIG. 10 may be used atall times or may be substituted for the method of FIG. 9 when a recentstylus location is not available. In FIG. 10 a candidate carrier of merenoise is arbitrarily selected in stage 146. Then detection and noisesubtraction are carried out over all elements of the digitizer on thebasis of the selection being correct, in stage 148. In stage 150 theresulting detection pattern is analyzed. In stage 152, a decision ismade as to whether the detection pattern in fact indicates a correctselection or not. If a correct selection is indicated then the resultsare accepted in stage 154, otherwise the results are rejected and a newarbitrary selection is made. In alternative or complimentaryembodiments, either unlikely patterns are rejected, or likely patternsare positively selected.

As will be explained in greater detail below, the different elements areat different spatial locations and may have different characteristics,as a result of whichthe same signals may be at different phases andmagnitudes. A preferred embodiment thus uses a complex proportion tocompensate for either or both of phase and magnitude differences.

In one variation the digitizer is a grid array of detection elements.One of the elements is selected as the candidate carrier of mere noiseusing any of the methods described above or other methods that willoccur to the skilled person, and then the outputs of either all or partof the other elements in the grid are reduced in accordance with thedetected noise in the candidate carrier of mere noise.

In another preferred embodiment, the method involves using, not justone, but a group of arbitrarily selected frequencies to calculate thereduction. The reduction calculation may thus be based on an averageoutput within the group.

As will be explained in greater detail below, the detecting elements maybe conductive detectors of a flat array. The detected object may be astylus but may alternatively be any moving object emitting locationsignals at a given frequency.

The noise is often at least in part a consequence of the presence of afinger.

As explained above, noise reduction is carried out by identifying noiselevels at the stylus frequency and subtracting them from the signalsreceived on the sensing elements or antennas sensing the stylus. To doso, the digitizer must first find an antenna exhibiting pure noise (i.e.an antenna that is least effected by the stylus), and then use theoutput at that antenna to calculate the noise components on thestylus-detecting antennas.

FIG. 11 is an illustration of the solution described above in an idealdigitizer system where the amplifiers are identical and have infiniteinput resistances. In particular the user induced signals do not suffermagnitude or phase shifts due to resistance of sensor conductors,differences in parasitic input capacitances and the like. This scenariocorresponds to the simplified flow chart described in FIG. 6.

FIG. 11 shows a sensor 220 comprising a grid of detecting elements,including elements 252 and 253. A finger is at location 222 and a stylusat location 221. A mixed output 251 is obtained from detecting element252 due to the finger and the stylus. However element 253 is notaffected by the finger.

In accordance with the above assumptions, a detected noise signalinduced by a user finger 222 touching two different antennas 252 and 253is similar. The stylus antenna 252 receives signals 251 from both fingerlocation 222 and stylus location 221. As long as both antennas 252 and253 are sampled simultaneously the signal 254 detected on the fingerantenna 253 can be subtracted from the mixed signal 251 received on thestylus antenna 252. As a result the pure stylus signal is obtained atthe stylus antenna 252.

A Solution for Non-Ideal Digitizers

Detecting Noise Signals

As explained above, a preferred implementation of the present inventionutilizes noise signals at an arbitrary frequency, different from thestylus frequency, in order to reduce noise signals in the stylusfrequency. For purposes of the present disclosure the frequency used fornoise signals that are not at the stylus frequency will be referred toas f_(arb). As described above, the digitizer preferably finds anantenna exhibiting pure noise signals in order to eliminate the noisesignal from the stylus antenna. One way to find a pure noise antennalies in the ability to detect noise signals in f_(arb).

The noise spectrum is usually wider then the stylus frequency range;thus sampling a range of frequencies around the stylus frequency is mostlikely to reveal noise signals at other frequencies as well. In thiscase each time the digitizer samples the antennas it scans a range offrequencies around the stylus frequency. Signals received in frequenciesother than the stylus frequency are evidently noise. In one embodiment,the digitizer selects a frequency showing the strongest noise level, aslong as it is above a certain threshold, to be the frequency f_(arb). Inother embodiments, the noise frequency can be selected according tovarious system considerations. For example, some embodiments mayimplement touch detection at one frequency (f₁) and stylus detection ata second frequency (f₂). Since the system is already configured toexamine signals at f₁, it may be preferable to utilize f₁ as the noisefrequency (f_(arb)=f₁).

In some cases it is possible to deliberately generate a noise signal ata predetermined frequency (f_(arb)) that is induced by the same sourceof unwanted noise. An example for such a case is a conductive object,such as a users finger, touching the screen and inducing noise signals.Thus, oscillating the digitizer's antennas intentionally will createfinger-induced signals at the frequency of the oscillating antennas. Theantenna oscillations may be at any frequency (f_(arb)) other than thestylus frequency. However, the best results are achieved when f_(arb) isclose to the stylus frequency. In this case f_(arb) is determinedaccording to the frequency utilized to generate the noise outputs asopposed to the first scenario where it is determined on the fly. Notethat in most applications the stylus may oscillate at more than onefrequency at different times. For example, when the stylus is hoveringabove the sensor it may be set to oscillate at one frequency (f₁) andwhen it is in contact with the sensor (‘tip-down’) it may be set tooscillate at a second frequency (f₂). The digitizer preferablyrecognizes the status of the stylus (i.e. hovering or ‘tip-down’)according to the frequency of the stylus signal. In this case, theselected noise frequency (f_(arb)) corresponds to the relevant stylusfrequency. When the stylus is in the hovering state, f_(arb) is close tof₁. When the stylus is in contact with the sensor a second noisefrequency is utilized, closer to f₂.

Finding the Pure Noise Antenna

It is reasonable to assume that in any case when noise sources arepresent, the system can identify at least one antenna that exhibits purenoise signals. This assumption is based on the fact that the stylus isaffecting a relatively small area in the proximity of its tip and thefact that physically the stylus and noise source can not be located atthe exact same place.

The preferred embodiments use two different methods in order to find apure noise antenna. The first method, described above with respect toFIG. 9, is applicable only when the previous location of the stylus isknown. FIG. 12 is a simplified diagram illustrating a digitizer on whichis carried out the method of FIG. 9, namely of finding the pure noiseantenna using the stylus' previous location. FIG. 12 shows the same gridarrangement as in FIG. 11, with a finger at one location and the stylusat another location. The method of FIG. 12 relies on the fact that thestylus movements across the sensor are continuous. In addition, theantennas sampling rate is such that the previous location of the stylusis a good indication of its current whereabouts. The following steps areused to identify the pure noise antenna:

1. Detecting antennas exhibiting noise signals in f_(arb)—This stage canbe preformed in any of the ways described hereinabove or any othermethods that may occur to the skilled person.

2. Ignore any antennas that might be used for stylus detection, such asantenna 261. The algorithm chooses the antenna farthest away from thestylus' previous location, but still exhibiting a noise signal exceedinga certain threshold. In this case antenna 262 is most likely to bechosen as it has a noise source located thereon, namely a finger.

Alternatively, the algorithm may choose the antenna exhibiting thestrongest noise signal as long as it is sufficiently distanced from thestylus previous location, again as described above.

The second method, as illustrated above in FIG. 10, does not rely on theprevious location of the stylus, thus it can be implemented even whenthe stylus is not present at the sensor surface or when a previouslocation is not known. The procedure of finding the noise antenna, usingthe second method is as follows

1. Detecting several antennas exhibiting a strong enough noise signal inf_(arb)—as described hereinabove.

2. Choose one of the antennas to calculate the noise component in thestylus frequency on all the other antennas, as described elsewhereherein. The choice may be made arbitrarily or using any suitablealgorithm.

3. If there is no stylus in the region of the sensor then the selectedantenna must exhibit a pure noise signal and after subtraction all othersignals will be very low. On the other hand, if there is a stylusdetected by the sensor, there are two options:

-   -   The selection was correct and the chosen antenna exhibit pure        noise signals. In this case the noise is subtracted from all        other antennas and the stylus is detected correctly.    -   The selection was wrong and the chosen antenna exhibits a mixed        output of noise and of stylus signal. In such a case, after        subtraction of the incorrect pure noise elements from all the        other antennas, the pattern of signals exhibited on the entire        sensor will not match the kind of pattern induced by a real        stylus. For example, a pattern may be considered invalid if the        number of antennas exhibiting stylus signals is above a certain        threshold. An invalid pattern can also be identified by the        distance between the stylus antennas, according to the spatial        EM field emitted from the stylus tip.

The system thus identifies an invalid pattern and goes on to select afurther candidate for being a pure noise antenna, preferably the antennaexhibiting the largest noise signal (f_(arb)) on the other axis. Noiseis subtracted from all other antennas and the system looks for a validstylus pattern. This process of antenna selection and noise subtractionis repeated until choosing a real pure noise antenna thus detecting avalid stylus pattern after noise subtraction.

Reference is now made to FIG. 13 which illustrates the grid of FIG. 11on which the method of FIG. 10 is being carried out. In FIG. 13 thedigitizer identifies antenna 268 which is the antenna exhibiting a largenoise signal on the X axis. The digitizer further identifies antenna 263exhibiting a large noise signal on the Y axis. FIG. 13 clearly showsthat only antenna 263 is exhibiting pure noise, output 264. Antenna 268by contrast exhibits a mixed signal 267 including elements from bothnoise source 265 and a stylus 266.

The algorithm chooses one of the noise antennas as a pure noise antennaand uses it to calculate the noise component in the stylus frequency onall the other antennas, as described hereinbelow. As will be explainedbelow, the antennas affected by these calculations are those exhibitingnoise signals in f_(arb). For example, the stylus antenna 261 isunaffected by the noise source, therefore its signals are not altered.

Now we review the case when the algorithm chooses the antenna 268exhibiting mixed signals 267 instead of the antenna 263 exhibiting purenoise 264. As a result of choosing the wrong antenna the digitizer willnot be able to determine the stylus position. Referring now to FIG. 14,we describe the signals received at antenna 263 and antenna 268 aftersubtracting the mixed signals 267 on the chosen noise antenna (68).Antenna 268 does not exhibit any signals in the stylus frequency whileantenna 263 exhibits a signal 269 at the stylus frequency. Theseindications imply that the stylus affects only one axis, and thus itslocation cannot be determined.

Since the stylus cannot in reality only affect a single axis anerroneous selection of the noise antenna is a clear conclusion. Thealgorithm thus proceeds to choose the noise antenna on the other axis asthe pure noise antenna.

For purposes of the present disclosure the antenna exhibiting pure noisesignals is referred to hereinbelow as the ‘Noise Antenna’.

When the previous location of the stylus is unknown, the noise removalalgorithm may consist of several iterations. In this case it would bepreferable to avoid unnecessary processing, and issue the noise removalalgorithm only when there is high certainty that a stylus is indeedpresent. Once an antenna exhibits a strong enough signal in the stylusfrequency, the digitizer compares the signals at the stylus frequency(f_(s)) to signals received at the noise frequency (f_(arb)). Thealgorithm checks the following ratio$\frac{{{Mag}\left( {f_{arb},X} \right)} \cdot {{Mag}\left( {f_{arb},Y} \right)}}{{{Mag}\left( {f_{s},X} \right)} \cdot {{Mag}\left( {f_{s},Y} \right)}}\overset{?}{>}{Threshold}$

Where, Mag(f_(arb),X) is the magnitude of the signal in the noisefrequency (f_(arb)), received on the antenna exhibiting the strongestsignal on the X axis.

Mag(f_(arb),Y) is the magnitude of the signal in the noise frequency(f_(arb)), received on the antenna exhibiting the strongest signal onthe Y axis.

Mag(f_(s),X)—The magnitude of the signal in the stylus frequency(f_(s)), received on the antenna exhibiting the strongest signal on theX axis.

Mag(f_(s),Y)—The magnitude of the signal in the stylus frequency(f_(s)), received on the antenna exhibiting the strongest signal on theY axis.

As stated above, the ratio is calculated using the antennas exhibitingthe highest signals on each axis. When the ratio exceeds a certainpredetermined threshold it means that the signals are most likelyoriginating from noise in the stylus frequency rather than a stylus. Inthis case the signals are preferably discarded.

Proportion Coefficient Between a Pair of Antennas

As explained above, when a signal is induced by a single source, thedigitizer may sense that signal in a different magnitude or phase ondifferent antennas. Some of the reasons for the differences in phase andmagnitude are:

-   -   The distance of the signal source form the different antennas    -   The fact that the amplifiers at the end of the antennas are not        necessarily identical, they may for example have different input        resistances.    -   The different locations of the signal source in respect to the        inputs to the amplifiers.

Since the preferred embodiments utilize the signal received on oneantenna to calculate the signal received on a second antenna, it isimportant to compensate for the variations in phase and magnitude. To doso a complex proportion coefficient is preferably used to correlatebetween signals received on the different antennas.

Reference is now made to FIG. 15 which is a simplified circuitequivalent showing two antennas, 273 and 277, being affected by a singleoscillating source 270 which represents a user finger. Each antenna 273,277, is connected to the oscillating source 270 through a different setof resistors and capacitors. Capacitor C1 271 represents the capacitancebetween the users finger 270, and antenna 273. Resistor R1 272,represents the internal resistance of antenna 273. Capacitor C2 275represents the capacitance between the user's finger 270 and antenna277. Resistor R2 276 represent the internal resistance of the secondantenna 277. Capacitors C3 278 and C4 274 represent the parasiticcapacitance between the antennas and the surrounding components. It willbe apparent that since the antennas have different characteristics andlocations they are affected differently by the oscillating source.

The signal at first antenna 273 may be expressed as—S₁=Z1·F

-   -   where F is the signal induced by the finger and Z1 is a complex        number representing the phase shift and magnitude change due to        capacitors C1 C4 and resistor R1. Z1 also incorporates the phase        shift and magnitude change due to the distance of the finger        from the first antenna 273.

The signal on the second antenna 277 may be expressed as—S₂=Z2·F

-   -   where F is the signal induced by the finger and Z2 is a complex        number representing the phase shift and magnitude change due to        capacitors C2 C3 and resistor R2. Z2 also incorporates the phase        shift and magnitude change due to the distance of the finger        from the second antenna 277.

Dividing the above equations gives a proportion coefficient (C=Z1/Z2)that can be used to reduce the signal on one antenna (S₁) based on thesignal received on a second antenna (S2) as follows: $\begin{matrix}{\text{Equation~~~~1:}\quad} \\{\frac{S_{1}}{S_{2}} = {\left. \frac{Z\quad 1}{Z\quad 2}\Rightarrow S_{1} \right. = {\left. {\frac{Z\quad 1}{Z\quad 2} \cdot S_{2}}\Rightarrow S_{1} \right. = {C \cdot S_{2}}}}}\end{matrix}$

Since Z1 and Z2 are complex numbers, their ratio is also a complexnumber representing the phase shift and magnitude difference between thesignals received in the first antenna 273 and the second antenna 277.

The phase shift is calculated by subtracting the phase part of Z2 fromthe phase part of Z1—The magnitude difference is calculated by the ratiobetween the magnitude parts of Z1 and Z2${{MAG}\left\{ C \right\}} = {\frac{{MAG}\left\{ {Z\quad 1} \right\}}{{MAG}\left\{ {Z\quad 2} \right\}}.}$

The proportion coefficient (C) is a function of many parameters such asthe distance between the antennas and the signal's source, theresistances of the antennas, envirornmental conditions, differentparasitic capacitances on each antenna etc. However, for a small enoughfrequency range the proportion coefficient is unchanged. Thus, the sameproportion coefficient can be used when sampling signals of closefrequencies on the same pair of antennas, at the same time. It is alsopossible to establish one proportion coefficient (C₁), corresponding toa first range of frequencies around f₁, from a second proportioncoefficient (C₂), corresponding to a second range of frequencies aroundf₂.

Referring now to FIG. 16, which is a simplified diagram showing twosources 280 and 281 oscillating at close but different frequencies, f₁and f₂, affecting a pair of antennas 273 and. 277. Parts that are thesame as in previous figures are given the same reference numerals andare not referred to again except as necessary for understanding thepresent embodiment.

The oscillating signals 280 and 281 oscillate at two different yetrelatively close frequencies—f₁ and f₂. The oscillating energy istransmitted to the antennas through the equivalent of a set of resistorsand capacitors as previously described in FIG. 16. Signals induced byfirst oscillator 280 are received on first antenna 273 as signal S₁(f₁)282 and as signal S_(2 (f) ₁) 283 on second antenna 277. Signals inducedby the second oscillator 281 are received on first antenna 273 as signalS₁(f₂) 284 and as signal S₂(f₂) 285 on second antenna 277. Theoscillating frequencies are such that the same proportion coefficientcan be used for signals at both frequencies.

As long as the above signals are sampled at the same time, a proportioncoefficient can be used for calculating signals received on the firstantenna by sampling the signals received on the second antenna:S ₁(f ₁)=C·S ₂(f ₁)   Equation 2:S ₁(f ₂)=C·S ₂(f ₂)   Equation 3:

Since the proportion coefficient is approximately the same for both ofthe above equations $\begin{matrix}{\text{Equation~~~~4:}\quad} \\{\frac{S_{1}\left( f_{1} \right)}{S_{1}\left( f_{2} \right)} = \frac{S_{2}\left( f_{1} \right)}{S_{2}\left( f_{2} \right)}}\end{matrix}$

The present invention uses Equation 4 in order to calculate the noisecomponent on the stylus frequency as will be elaborated hereinbelow.

Subtracting the Noise Component from the Stylus Signal

The present embodiments may be implemented on any antenna, whether ornot it exhibits a stylus signal. It is possible to calculate the purenoise signal on each and every antenna and subtract it from the overallsignal received on said antenna. When a stylus signal is indeed present,the result will be a pure stylus signal. When the antenna is unaffectedby the stylus the subtraction of the pure noise signal will indicatethat no signals are present on the antenna. In both cases precisedetection of the stylus is achieved.

The above principle is now explained using one antenna exhibiting mixedstylus and noise signals and another antenna exhibiting pure noisesignals. Reference is now made to FIG. 17, which is a graph showingintensity vs. frequency of the signals sampled on one of the antennassensing mixed stylus and noise signals. This antenna is now referred toas the stylus antenna.

The pure noise signal in f_(arb) received on the stylus antenna ismarked N′(f_(arb)) 300. The pure noise component induced by the noisesource in the stylus frequency is N′(f_(s)) 301. The pure stylus signalis marked S(f_(s)) 302. Noise removal according to the presentembodiments comprises distinguishing between N′(f_(s)) and S(f_(s)).Note that the noise reduction algorithm can be applied even ifS(f_(s))=0.

Reference is now made to FIG. 18 which describes the signals sampled onan antenna that is unaffected by the stylus. This antenna is referred toas the pure noise antenna, since it exhibits noise signals alone. Thesignal received on the pure noise antenna in f_(arb) is markedN(f_(arb)) 305. The pure noise signal received on the noise antenna inthe stylus frequency is marked N(f_(s)) 304.

The intensity of the signals is arbitrary, and used for illustrating thekinds of signals received on each antenna. The present embodimentsidentify N′(f_(s)) 301 in order to subtract it from the signals receivedon the antenna sensing the stylus

FIGS. 17 and 18 describe stylus induced signals on one specific antenna.However, the method can be implemented on any antenna or group ofantennas.

Signals N(f_(arb)), N′(f_(arb)) and N(f_(s)) are sampled simultaneously,hence (based on equation 4) $\begin{matrix}{\text{Equation~~~~5:}\quad} \\{\frac{N\left( f_{arb} \right)}{N^{\prime}\left( f_{arb} \right)} = \frac{N\left( f_{s} \right)}{N^{\prime}\left( f_{s} \right)}}\end{matrix}$

The frequency chosen for detecting the noise signals is preferablydifferent from the frequency currently used by the stylus, yet closeenough for equation 4 to remain valid.

Notice that the only unknown parameter in equation 5 is N′(f_(s)),therefore $\begin{matrix}{\text{Equation~~~~6:}\quad} \\{{N^{\prime}\left( f_{s} \right)} = {\frac{N^{\prime}\left( f_{arb} \right)}{N\left( f_{arb} \right)} \cdot {N\left( f_{s} \right)}}}\end{matrix}$

Once N′(f_(s)) is calculated it is subtracted from the signal receivedon the stylus antenna revealing the signal induced by the stylus aloneS(f_(s)).

Improving the Proportion Coefficient Calculations

The preferred embodiments use the ratio between detections at the purenoise antenna and detections received on other antennas at differentfrequencies in order to calculate the noise component on the stylusfrequency as explained.

Averaging Over Several Arbitrary Frequencies

The embodiments of the present invention described above use onearbitrary frequency in order to calculate the above ratio. However, inan alternative embodiment several frequencies are used, and an averageis taken of the detections induced on the respective antennas for thedifferent frequencies. The use of more then one arbitrary frequencyreduces the proportion coefficient dependence on frequency.

The digitizer according to this embodiment thus uses different arbitraryfrequencies (f¹ _(arb),f² _(arb), . . . ,f^(n) _(arb)) in order todetect noise signals in several frequencies. The noise signalsthemselves are detected as described hereinabove.

Sampling the conductive lines provides detections at frequencies—f¹_(arb), f² _(arb), . . . , f^(n) _(arb).

The average output received on a stylus antenna due to finger noisealone is${{\overset{\_}{N}}^{\prime}\left( f_{arb} \right)} = {\frac{\sum\limits_{i = 1}^{n}\quad{N^{\prime}\left( f_{arb}^{i} \right)}}{n}.}$

The average output received on the noise antenna due to finger noise is${\overset{\_}{N}\left( f_{arb} \right)} = {\frac{\sum\limits_{i = 1}^{n}\quad{N\left( f_{arb}^{i} \right)}}{n}.}$

Since all detections are sampled simultaneously, the ratio between theaveraged signals and the signals received at the stylus frequency is$\begin{matrix}{\text{Equation~~~~7:}\quad} \\{\frac{\overset{\_}{N}\left( f_{arb} \right)}{{\overset{\_}{N}}^{\prime}\left( f_{arb} \right)} = \frac{N\left( f_{s} \right)}{N^{\prime}\left( f_{s} \right)}}\end{matrix}$

Once again, the only unknown parameter is N′(f_(s)), hence$\begin{matrix}{\text{Equation~~~8:}\quad} \\{{N^{\prime}\left( f_{s} \right)} = {\frac{{\overset{\_}{N}}^{\prime}\left( f_{arb} \right)}{\overset{\_}{N}\left( f_{arb} \right)} \cdot {N\left( f_{s} \right)}}}\end{matrix}$

The stylus signal, S(f_(s)), is then calculated by subtracting N′(f_(s))from the signal received on the stylus antenna.

It is expected that during the life of this patent many relevant stylusdevices and digitizer systems will be developed and the scope of thecorresponding terms herein, is intended to include all such newtechnologies a priori.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1. A method for noise reduction in a digitizer, the digitizer comprising a plurality of detecting elements for detecting an electromagnetic signal at one of a number of predetermined frequencies: the method comprising: sampling at least two of said detecting elements substantially simultaneously to obtain outputs therefrom, and reducing the output on one of said two elements in accordance with the output on the other of said elements.
 2. The method of claim 1, wherein one of said at least two elements is selected as a candidate carrier of a stylus signal and the other of said at least two elements is selected as a candidate carrier of mere noise.
 3. The method of claim 2, comprising detecting at each of said elements said one predetermined frequency and another arbitrary frequency.
 4. The method of claim 3, comprising utilizing said arbitrary frequency to estimate the amount of noise at said predetermined frequency.
 5. The method of claim 3, comprising: determining a ratio between signals of the one predetermined frequency and the arbitrary frequency at said candidate carrier of mere noise, from said ratio and the arbitrary signal at the candidate carrier of the stylus signal determining an amount of noise at the one predetermined frequency, and reducing a signal at said one predetermined frequency at said candidate carrier of a stylus signal by said determined amount of noise.
 6. The method of claim 5, wherein said arbitrary frequency is selected as a frequency within a preset detection range having a relatively high noise.
 7. The method of claim 5, wherein said digitizer is also for touch detection and said arbitrary frequency is selected as a frequency already used for said touch detection.
 8. The method of claim 5, further comprising deliberately generating noise at said arbitrary frequency.
 9. The method of claim 5, wherein said predetermined frequency is changed to a new frequency during use, the method comprising changing said arbitrary frequency from a frequency relatively close to said predetermined frequency to a second frequency relatively close to said new frequency.
 10. The method of claim 2, wherein said candidate carrier of mere noise is selected from a group of elements exhibiting more than a threshold amount of noise, as the element in said group which is furthest away from a stylus previously known location.
 11. The method of claim 2, wherein said candidate carrier of mere noise is selected as an element exhibiting a strongest noise signal as long as it is beyond a determined distance from a stylus previously known location.
 12. The method of claim 2, wherein: said candidate carrier of mere noise is selected, noise subtraction is carried out over a group of elements on the basis of said selection being correct, a resulting signal pattern over said group of elements is analyzed, and said arbitrary selection is allowed if said resulting signal pattern is indicative of a correct selection, otherwise a new arbitrary selection is made.
 13. The method of claim 12, comprising using as patterns indicative of a correct selection a first pattern indicative of a stylus at one of non-selected elements, and a second pattern indicative of no stylus being present.
 14. The method of claim 12, comprising verifying the presence of a stylus before carrying out said reduction.
 15. The method of claim 1, comprising verifying the presence of a stylus, beforecarrying out said reduction, by comparing magnitudes at the arbitrary frequency and magnitudes at the predetermined frequency.
 16. The method of claim 2, wherein: said candidate carrier of mere noise is selected, noise subtraction is carried out over a group of antennas on the basis of said selection being correct, a resulting signal pattern over said group of antennas is analyzed, and said arbitrary selection is rejected if said resulting signal pattern is indicative of an incorrect selection, and a new arbitrary selection is made.
 17. The method of claim 16, comprising using as a pattern indicative of an incorrect selection a pattern indicative of a stylus at or near said selected candidate.
 18. The method of claim 1, comprising using a complex proportion to compensate for at least one of phase and magnitude differences between respective antennas during said compensating.
 19. The method of claim 1, comprising sampling a group of antennas of an array, selecting at least one antenna which is least affected by the stylus and reducing respective outputs of at least some remaining antennas in accordance with the output of said selected antenna.
 20. The method of claim 3, comprising using a plurality of arbitrarily selected frequencies to calculate said reduction.
 21. The method of claim 20, comprising using an average output of said plurality of arbitrarily selected frequencies to calculate said reduction.
 22. The method of claim 1, wherein said detecting elements are conductive detectors of a flat array of said digitizer for digitizing signals of a movable object to indicate location of said object.
 23. The method of claim 22, wherein said noise is at least in part a consequence of the presence of a finger.
 24. The method of claim 1, wherein said digitizer comprises a detection surface for carrying said detecting elements, said detecting surface further being touch sensitive.
 25. Apparatus for noise reduction in a digitizer, the digitizer comprising a plurality of detecting elements for detecting an electromagnetic signal at one predetermined frequency of a plurality of predetermined frequencies: the apparatus comprising: a sampler for sampling at least two of said detecting elements substantially simultaneously to obtain outputs therefrom, and a noise reduction unit for reducing the output on one of said two detecting elements in accordance with the output on the other of said detecting elements.
 26. Apparatus according to claim 25, wherein one of said detecting elements is selected as a candidate carrier of a stylus signal and the other of said detecting elements is selected as a candidate carrier of mere noise.
 27. Apparatus according to claim 26, comprising a frequency detector for detecting outputs at each of said detecting elements at said predetermined frequency and another arbitrary frequency.
 28. Apparatus according to claim 27, comprising: a ratio finder for determining a ratio between outputs at the predetermined frequency and the arbitrary frequency at said candidate carrier of mere noise, and wherein said noise reduction unit is operable with said ratio finder to: determine from said ratio and the arbitrary signal at the candidate carrier of the stylus signal determining an amount of noise at the predetermined frequency, and reduce the output at said predetermined frequency at said candidate carrier of a stylus signal by said determined amount of noise.
 29. Apparatus according to claim 27, wherein said detecting elements are transparent conductors.
 30. Apparatus according to claim 28, wherein said arbitrary frequency is selected as a frequency within a preset detection range having a relatively high noise.
 31. Apparatus according to claim 28, wherein said arbitrary frequency is selected as a frequency already used for finger detection.
 32. Apparatus according to claim 28, further comprising a noise generator for deliberately generating noise at said arbitrary frequency
 33. Apparatus according to claim 28, wherein said predetermined frequencies are liable to change during use, the apparatus accordingly being configured to change said arbitrary frequency from a frequency relatively close to a first predetermined frequency to a second frequency relatively close to a second predetermined frequency.
 34. Apparatus according to claim 26, wherein said candidate carrier of mere noise is selected from a group of detection elements exhibiting more than a threshold amount of noise, as the element in said group which is furthest away from a stylus previously known location.
 35. Apparatus according to claim 26, wherein said candidate carrier of mere noise is selected as an element exhibiting a strongest noise signal as long as it is beyond a determined distance from a stylus previously known location.
 36. Apparatus according to claim 26, wherein: said candidate carrier of mere noise is selected, noise subtraction is carried out over a group of elements on the basis of said selection being correct, a resulting signal pattern over said group of elements is analyzed, and said arbitrary selection is allowed if said resulting signal pattern is indicative of a correct selection, otherwise a new arbitrary selection is made.
 37. Apparatus according to claim 36, comprising using as patterns indicative of a correct selection a first pattern indicative of a stylus at one of non-selected elements, and a second pattern indicative of no stylus being present.
 38. Apparatus according to claim 36, wherein said analysis comprises determining a number of elements wherein an output exceeds a predetermined threshold.
 39. Apparatus according to of claim 26, wherein: said candidate carrier of mere noise is selected, noise subtraction is carried out over a group of elements on the basis of said selection being correct, a resulting signal pattern over said group of elements is analyzed, and said arbitrary selection is rejected if said resulting signal pattern is indicative of an incorrect selection, and a new arbitrary selection is made.
 40. Apparatus according to claim 25, comprising at least one of a phase compensator and a magnitude compensator using a complex proportion to compensate for at least one of phase and magnitude differences between respective elements during said compensating.
 41. Apparatus according to claim 25, wherein said elements comprise an array, the apparatus configured to sample at least some elements of said array, said noise reduction unit being configured to choose at least one element and reduce respective outputs of at least some remaining elements in accordance with the output of another of said elements.
 42. Apparatus according to claim 27, wherein said noise reduction unit is configured to use a plurality of arbitrarily selected frequencies to calculate said reduction.
 43. Apparatus according to claim 42, wherein said noise reduction unit is configured to use an average noise level of said plurality of arbitrarily selected frequencies to calculate said reduction.
 44. Apparatus according to claim 25, wherein said elements are conductive detectors of a flat array of said digitizer for digitizing signals of a movable object to indicate location of said object.
 45. Apparatus according to claim 44, wherein said movable object is a stylus.
 46. Apparatus according to claim 45, wherein said stylus is an electromagnetic stylus.
 47. Apparatus according to claim 46, wherein said stylus comprises an active oscillator and an energy pick up circuit.
 48. Apparatus according to claim 46, wherein said stylus comprise a resonance circuit.
 49. Apparatus according to claim 25, wherein said detecting elements are arranged in a grid array.
 50. Apparatus according to claim 25, wherein said detecting elements are loop elements.
 51. Apparatus according to claim 44, wherein said noise is at least in part a consequence of the presence of a finger. 